1. Field of the Invention
This invention relates generally to radiographic systems and is concerned more particularly with a radiographic system having means for permitting an operator to monitor dynamic operation of the X-ray source while studying the output radiographic image.
2. Discussion of the Prior Art
A radiographic system generally includes an X-ray source disposed for directing an X-ray beam through a selected portion of a patient and transmitting an X-ray image thereof to a suitable image receptor. The image receptor may comprise an image converter, such as an image intensifier tube, for example, which receives the X-ray image and produces a corresponding output visible image. Thus, by suitable protective viewing means, such as television or optics, for examples, a radiologist may study the output visible image while the patient is being irradiated.
The X-ray source generally comprises an X-ray tube having an evacuated envelope wherein an electron emitting cathode is disposed for directing a beam of electrons onto a focal spot area of an anode target. In operation, the cathode supplies an electron current which is electrostatically focused onto the focal spot area by means of a high voltage impressed between the cathode and the anode. The electrostatically accelerated electrons impinge on the focal spot area with sufficient kinetic energy to generate the beam of X-rays which passes through the selected portion of the patient.
However, only about one percent of the input energy comprising the electron beam current and the electron accelerating voltage is converted into X-radiation. The remainder of the input energy is converted into thermal energy which greatly increases the temperature of the anode target. Generally, this thermal energy is stored in the mass of the anode and is dissipated by thermal radiation to the surrounding tube envelope. If the rate of increase in target temperature or the heat storage capacity of the anode exceed critical values, melting and cracking of anode target may occur, thereby reducing X-ray output and possibly rendering the tube inoperative.
Consequently, means have been developed for monitoring the temperature of the anode target during operation of the tube and indicating when the temperature is approaching a critical value. Thus, the X-ray tube may be deenergized temporarily to allow the anode to cool and thereby prolong the life of the tube. However, the temperature indicator of the monitoring means may be located in a position where it is not readily observable during an X-ray exposure, particularly when subdued illumination is required. Furthermore, during an X-ray exposure, the radiologist is primarily interested in the patient and, therefore, is concentrating on the output visible image. As a result, the anode target temperature may exceed the critical value, without being noticed, during the X-ray exposure.
Therefore, it is advantageous and desirable to provide a radiographic system with means for permitting a radiologist to monitor the anode target temperature while studying the output visible image produced by the radiographic system.